Thermography is an image-forming process including a heating step and hence includes photothermography in which the image-forming process includes image-wise exposure and direct thermal processes in which the image-forming process includes an image-wise heating step.
In direct thermal printing a visible image pattern is produced by image-wise heating of a recording material e.g. image signals can be converted into electric pulses and then via a driver circuit selectively transferred to a thermal head, which consists of microscopic heat resistor elements, thereby converting the electrical energy into heat via the Joule effect. This heat brings about image formation in the substantially light-insensitive thermographic material. In thermal heads, only those regions which produce heat higher than a certain value are effective for printing, and the regions capable of generating sufficient heat for the printing spread in proportion to voltage applied to the heating resistors. If, therefore, higher voltage is applied to the heating resistors, the size of the printing dots increases in proportion.
U.S. Pat. No. 6,462,766 discloses a method and an apparatus for limiting the peak power consumed by a thermal recorder connected to portable battery-powered equipment. The battery-powered equipment is designed with a filter and an electronic circuit breaker. A circuit breaker current sense resistor and an output capacitor form an RC filter and provide a large current reservoir for the thermal recorder which averages the peak current demands seen at the circuit input. The electronic circuit breaker provides a current limit function and will not allow a current greater than a predetermined amperage level to be drawn. The thermal recorder has a CPU which provides pulses to a thermal print head in dependence on data incorporated in a pulse-width limit table. The values in the pulse-width limit table can be substituted for calculated pulse widths that would produce peak currents large enough to trip the circuit breaker.
U.S. Pat. No. 6,234,695 discloses a printer using a power reduction logic based upon reducing the speed of printing when the dot utilization calculation exceeds a particular power level for that printer. There is also provided a method for printing information at a given power supply capacity level, comprising the steps of: examining the a group of rows of dots to be printed; calculating the maximum dot utilization value for the group; selecting a print speed based on the maximum dot utilization value; printing the first row of the group of rows; and repeating above steps until the information is printed.
U.S. Pat. No. 6,503,006 discloses a printer using a power reduction logic based upon reducing the speed of printing when the dot utilization calculation exceeds a particular power level for that printer. There is also provided a method for printing information at a given power supply capacity level, comprising the steps of: examining the a group of rows of dots to be printed; calculating the maximum dot utilization value for the group; selecting a print speed based on the maximum dot utilization value; printing the first row of the group of rows; and repeating above steps until the information is printed.
U.S. Pat. No. 5,528,275 discloses a gradational printing method for performing a gradational printing by energizing a plurality of heat emitting elements arranged on a thermal head correspondingly to respective bits of digital gradation data representing a gray level. In case of this method, the plurality of heat emitting elements are divided into two or more blocks and the blocks are energized correspondingly to different bits of the digital gradation data. Thus, an energizing time, during which a maximum current should be supplied, can be reduced.
EP-A 0 453 714 further discloses a gradation record printer in which an amount of energy applied to a thermal head is controlled in response to a gradation level of an image signal to print an image on a printing medium with gradations, said printer comprising: gradation density detecting means for storing data of standard density patterns with respect to address values corresponding to gradation levels, and for outputting a coincidence signal when data supplied from an outside of said gradation density detection means substantially coincides with said data of standard density patterns; a gradation test print circuit for applying data of different amounts of printing energy to said thermal head sequentially to make a gradation test print on said printing medium; density detecting sensor means for detecting densities of said gradation test print and for applying a detection output from said density detecting sensor means to said gradation density detection means; and a first memory element for storing said data of different amounts of energy with respect to address values corresponding to said gradation levels, in response to said coincidence signal from said gradation density detecting means.
EP-A 1 247 654 discloses that the traditional technique for calibrating a thermal printer is as follows: first, a first calibration page is printed with a limit setting to produce the desired maximum density and a full range of print settings. The next step is to determine whether this is the desired limit setting by visually inspecting the printed page. The normal objective is to find the minimum exposure required to print the full range of desired densities. The lower the limit setting, the more nearly continuous the grey scale in the printed film. The process of printing and adjusting the maximum limit setting is repeated until a desired limit setting is determined. Next, a second calibration page is printed with the limit system setting selected and with a subset of print system settings which cover the full range of print settings. The resulting densities of the printed page are then measured and a print setting to density table created for the full range of print settings. An output lookup table that can be used to set exposure to produce the desired density for any digital image value is created using the print setting to density table. Thereafter the thermal printer prints pages with this output lookup table to produce the desired densities while the same maximum exposure is appropriate.
EP-A 1 247 654 further discloses a method for calibrating a thermal printer comprising a thermal head incorporating a plurality of energisable heating elements, said method comprising the steps of: supplying to said thermal printer a thermographic material m, a plurality of printer data Pi each intended to be recorded as a pixel having a density Di, and default reference values for printing parameters II comprising a value Pref for a reference printing power; printing a calibration pattern for said plurality of printer data Pi, said calibration pattern comprising a multiple step density wedge such that a whole range of a relation Di (Pi) between said printer data Pi and said density Di is covered; measuring a density Dexpi for each patch of said density wedge of said calibration pattern in relation to said plurality of printer data Pi and storing a first set S1=(Pref, Pi, Dexpi) in a first memory M1; calculating, for a desired density Dwantj, a corresponding value Prefnewj for said reference printing power and storing a second set S2=(Dwantj Prefnewj) in a second memory M2; calculating, for said desired density Dwantj, for each printer data Pi a corresponding density Di and storing a third set S3=(Dwantj, Prefnewj, Pi, Di) in a third memory M3.
U.S. Pat. No. 5,711,621 discloses in a printer having a print region is defined by a thermal print head and a rotatable platen adapted to draw a print media therebetween, a method for calibrating the printer comprises the steps of: identifying a type of print media which is selected for use on said printer, wherein said selected type of print media has unknown printer parameter values; printing a series of test labels onto said selected type of print media using a parameter of said printer having a unique value for each individual one of said test labels of said series with identifying information of said unique parameter value being printed thereon; after printing said series of test labels, inspecting said series of test labels to select one of said test labels of said series having a desired level of image quality; and specifying said unique parameter value of said selected one of said test labels for further operation of said printer with said selected type of print media.
US 2001/004284 discloses a calibration pattern printing method for a printer that prints an image in an image recording area on a recording paper, comprising the steps of: printing a calibration pattern in a marginal area on the recording paper outside said image recording area; and cutting said marginal area with said calibration pattern off the recording paper after the image is printed in said image recording area.
Thermographic materials are increasingly being used for graphic arts, medical and other applications which require high maximum print densities. Attaining such high print densities in thermographic materials requires that the heating elements be driven at higher powers and hence to higher temperatures, which increases the probability of premature heating element failure due to overheating and of image faults in the thermographic materials due to overheating. A means is therefore required to avoid such failure of the heating elements due to overheating and to avoid such image faults in the thermographic materials without significant loss in maximum print density and significant loss in image information.